The demand for biosensors is increasingly growing these days. Usually, biosensors allow for the detection of a given specific molecule within an analyte, wherein the amount of said molecule is typically small. For example, one may measure the amount of drugs or cardiac markers within saliva or blood. Therefore, target particles, for example super-paramagnetic label beads, are used which bind to a specific binding site or spot only, if the molecule to be detected is present within the analyte. One known technique to detect these label particles bound to the binding spot is frustrated total internal reflection (FTIR). Therein, light is coupled into the sample at an angle of total internal reflection. If no particles are present close to the sample surface, the light is completely reflected. If, however, label particles are bound to said surface, the condition of total internal reflection is violated, a portion of the light is scattered into the sample and thus the amount of light reflected by the surface is decreased. By measuring the intensity of the reflected light with an optical detector, it is possible to estimate the amount of particles bound to the surface. This allows for an estimate of the amount of the specific molecules of interest present within the analyte or sample.
This technique as well as other magnetic-label sensors, in particular biosensors, critically depend on the magnetic attraction of the beads or magnetic labels, also referred to as actuation. Magnetic actuation is in particular essential in order to increase the performance (speed) of the biosensor for point-of-care applications. The direction of the magnetic actuation can be either towards the surface or sensor area where the actual measurement is carried out or away from this sensor surface. In the first case, magnetic actuation allows for the enhancement of concentration of magnetic particles near the sensor surface, thus speeding up the binding process of the magnetic particles to the sensor area. In the second case, particles are removed from the surface which is called magnetic washing. Magnetic washing can replace the traditional wet washing step. It is more accurate and reduces the number of operating steps.
In more complex applications, several binding spots may be provided on a tiny surface. It may then be necessary to first accumulate the particles or labels at a first binding site and after a washing step to drive the magnetic labels towards another binding site. Such applications afford a large amount of control of the magnetic field generated in order to provide precise and predetermined forces onto the magnetic label particles.